1. Field of the Invention
This invention relates to a process for preparing .gamma.-lactone derivatives. More particularly, this invention relates to a novel process for preparing .gamma.-lactone derivatives having the formula (I) ##EQU2## wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5 and R.sup.6, which may be the same or different, each represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms or a phenyl group which may be substituted with a halogen atom, and R.sup.1 and R.sup.2, and R.sup.5 and R.sup.6 may, when taken together with the carbon atom to which they are attached, form a cycloalkyl group having 4 to 7 carbon atoms, and R.sup.7 represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, a cycloalkyl group having 4 to 6 carbon atoms or a phenyl group which may be substituted with an alkyl group having 1 to 3 carbon atoms.
2. Description of the Prior Art
It is well known that .beta.-isobutenylisohexano-.gamma.-lactone (pyrocin) represented by the formula ##EQU3## can be obtained by dry distillation of pyrethrum flowers (Chrysanthemum cinerariaefolium). Hitherto, it has been reported in the literature that pyrocin can be prepared by synthetic methods, for example, the method comprising reacting an acid chloride of terebic acid (i.e., .gamma.,.gamma.-dimethylparaconic acid) with isopropyl iodide [Matsui et al., Bull. chem. Soc., Japan, 25, 210-214 (1952)], the method comprising pyrolysis of chrysanthemic acid in vacuo [L. Crombie et al., J. Chrm. Soc., 470 (1954)], the method comprising reacting isobutenylacetone with ethyl bromoacetate and methyl magnesium iodide [M. Julia et al., Compt. Rend., 251, 249-251 (1960)]. However, these known methods utilize starting materials which are extremely expensive and are not easily available and, further, the yield of the desired product, pyrocin, is said to be very low.
Also, it has been reported that pyrocin can be converted into crysanthemic acid or non-isoprenoid-type terpene alcohols by ring-opening of the lactone ring of pyrocin [Matsui et al., Agr. Biol. Chem., 26, No. 8, 532-534 (1962) and W. Sucrow et al., Tetrahedron Letters, No. 42, 3675-3676 (1970)] and pyrocin per se possesses an insecticidal activity.